My guess is there is a setting on your providers web site to enable historical emails. There certainly such a setting for GMail accounts and Yahoo offers different server settings for accounts over 10,000 email. They consider getting that many emails to be some form of archiving. So there is nothing in Thunderbird, but you probably should contact your provider. I see nothing mentioned on their email settings page about limits to old mail. https://selfserve.cablevision.qc.ca/mailguide/ Do you see these older email when you log in using a web browser? or only in another mail application or on a phone? It might be they are not on the server to get if say they are only visible in another mail application.